1. Field of the Invention
The present invention relates to a variable speed rotary drive mechanism. More specifically, the present invention includes a drive mechanism useful in operating prismatic display sign elements so as to vary the speed of rotation from a maximum velocity occurring during the changing of display surfaces to a minimum velocity occurring as the position corresponding to the display of the desired surface is approached.
2. Description of Related Technology
There are several known kinds of convertible signs, one type of which creates a display arrangement comprising prisms, each of which are mounted at opposite longitudinal ends and rotatable about their longitudinal axis. Each prism is part of an equal sequence in a frame, the prisms residing beside each other. The sides of the prism are oriented in a permanently occurring sequence forming a number of displays corresponding to the number of side surfaces of the single prism. Such a sign necessarily includes a drive motor for synchronous rotation of the prisms via a transmission.
Displays of this type usually comprise triangular aluminum prisms, which rotate in an aluminum frame and within 15 seconds show three displays in a permanently recurring sequence. The prisms can be dismounted and exchanged. Due to the triangular shape of the prisms, three different views can be shown. The display arrangement can be mounted with its frame standing free, on walls or on roofs of buildings.
Known display arrangements of this type are typically driven by an electric motor, and the prisms are rotated synchronously by a gear transmission in such a manner that the sides of the respective prisms belonging to the same picture are shown simultaneously and form a display. The gear transmission ensures synchronous rotation of each prism.
A gear transmission, however, involves certain disadvantages. A desirable characteristic of most convertible signs is that they operate noiselessly, particularly when the displays are mounted on buildings. A gear transmission for operating noiselessly requires good lubrication. This requirement would be difficult to attend to at many sign locations, because of the need for periodic service and such signs in most cases are positioned in places of difficult access. The problem of access has created a need for mechanisms of high durability and reliability, along with the continuing requirement for accurate registration of the sign elements and the constant need to start, rotate and stop the sign display. Experience in the field has demonstrated the need to convert this inherently complex mechanical operation into one having as much reliability and simplicity as possible.
One approach has been to opt for the relative simplicity and reliability of rotary motion, yet to ensure accurate registration of the sign elements by utilizing some sort of reciprocating element defining absolute limits of travel coincident with proper sign element registration. For example, U.S. Pat. No. 606,524, issued to Hopewell discloses reciprocating rods or bars which attach at one end to a drum or cylinder and at the other end to a peripheral area of a prismatic member. The periodic rotation of the cylinder causes the rods to physically push or pull an edge of each prismatic member, thereby causing each member to rotate about its longitudinal axis.
U.S. Pat. No. 1,718,625, issued to Bartoletti, discloses a rotary motor coupled to a reciprocating bar which in turn drives a bell crank attached to a ratcheting mechanism. The ratchet, or rack gear arrangement periodically rotates triangular members through 120.degree. of travel.
U.S. Pat. No. 2,360,780, issued to McCarty et al., discloses a rotating disk attached to an eccentrically mounted pin, each rotation of the disk causing a 90.degree. rotation of a rectangular sign face member.
U.S. Pat. No. 3,335,513, issued to Mann, discloses a hydraulic system which converts rotary to reciprocating motion.
U.S. Pat. No. 4,537,087, utilizes a pin mounted to a moving chain to travel within a slot on a pivotable arm. As the pin reciprocates within the slot, a rotary motion is imparted to the pivotable arm which thereby drives an output shaft at variable speed.
Other examples of belt or chain drives are disclosed in U.S. Pat. No. 4,759,140, issued to Roberts et al., in which a linear drive magnetic motor causes a belt to reciprocate between two positions, thereby rotating display panels through 180.degree.. Also, U.S. Pat. No. 4,987,691, issued to Kessels et al., discloses a belt drive for triangular sign face members using a belt drive which is linked to each prismatic member by means of a flexible metallic gripping member which frictionally engages the corner of each prismatic member so as to cause rotation of the prismatic member through an angle of 120.degree..
All of the aforementioned devices suffer from considerable mechanical complexity leading to substantial wear, parts failure, need for lubrication, and other preventive and periodic maintenance. A simpler solution from a mechanical standpoint is to limit the mechanical linkages solely to rotary motion, thereby avoiding the complexity of converting reciprocating motion to rotary motion and vice versa.
For example, U.S. Pat. No. 1,461,047, issued to Ray, uses a bevel gear attached to a motor to drive a mating bevel gear at the bottom of each triangular member. There are circumferential gaps in the teeth of the driving bevel gear, thereby causing the driven triangular member to stop periodically during the rotation of the driving gear.
U.S. Pat. No. 2,277,323, issued to Hjermstad et al., discloses a bevel gear arrangement in which the driving gear is activated periodically by cams connected to the output shaft of a motor. The 120.degree. spacing of the cams causes the triangular sign member to rotate through 120.degree. as well.
U.S. Pat. No. 2,839,855, issued to Palmer et al., discloses a two-sided sign in which a cam activates a mating lever for every 180.degree. of driving motor shaft rotation. A spring restrains the lever to a registered position when not driven by the cam member. The force of the motor and the driving cam is sufficient to overcome the resistance of the spring and permit another 180.degree. increment of sign rotation.
U.S. Pat. No. 4,638,580, issued to Giannetti et al., also discloses angularly spaced cams placed at 120.degree. intervals along a driving shaft which interact with mating pins on the driven shaft which is in turn connected to the triangular sign face members.
Specialized gears have been adapted to convertible sign applications, most notable among them being the so-called "Geneva" gear. A Geneva gear is a gear that normally includes a plurality of radially extending slots that coact with a pin or finger mechanism that is moved by another part of the mechanical system. As the pin coacts with one of the driving slots, the Geneva gear is rotated through a predetermined arc. Thereafter, a second driving slot coacts with the pin mechanism to rotate the Geneva gear through a second arc. The Geneva gear is stationary between rotational movement periods, which is ideal for a convertible sign application. An example of a Geneva gear device is disclosed in U.S. Pat. No. 3,675,503, issued to Upadhyay.
Another specialized gear, the "Maltese cross" and gear arrangement, has also been adapted to convertible signs, as disclosed in U.S. Pat. No. 3,921,321, issued to Weisskopf. Another type of indexing gear is utilized in U.S. Pat. No. 4,189,859, issued to Ahlgren, for a convertible sign application.
A typical problem of the aforementioned devices is undesired shock at the beginning and end of the motion. This is a particular problem with convertible signs which are often heavy and yet must stop and start several times each minute. One attempt to solve this problem is disclosed in U.S. Pat. No. 4,756,203, issued to Matsuda, which discloses a drive mechanism using a non-circular drive gear and elliptical cam.